In recent years, energy use has become more and more efficient, and demands are increasingly being made for reducing energy loss in transformers in operation. There are mainly two types of loss that may arise in transformers: one is copper loss in conductors and the other is iron loss in iron cores. Iron loss can be further divided into hysteresis loss and eddy current loss. Improving crystal orientations in the material and reducing impurities therein are known to be effective for reducing hysteresis loss.
For example, JP20121741A (PTL 1) describes a method for manufacturing a grain-oriented electrical steel sheet that shows excellent magnetic flux density and iron loss properties by optimizing the conditions of annealing prior to final cold rolling.
On the other hand, eddy current loss is known to drastically improve by reducing the sheet thickness, increasing the a mount of Si added, forming grooves on a surface of the steel sheet, and introducing strains.
For example, JPH0622179B (PTL 2) describes a technique to form linear grooves on one surface of a steel sheet with the groove width of 300 μm or less and the groove depth of 100 μm or less, whereby iron loss W17/50 can be reduced from at least 0.80 W/kg prior to the formation of grooves to 0.70 W/kg or less.
In addition, JP2011246782A (PTL 3) describes a technique whereby a steel sheet after being subjected to secondary recrystallization is subjected to plasma arc irradiation to reduce iron loss W17/50 from at least 0.80 W/kg prior to the irradiation to 0.65 W/kg or less.
Further, JP201252230A (PTL 4) describes a technique to obtain a material for transform ers that exhibits low iron loss properties and produces less noise by optimizing the film thickness and the average width of the magnetic domain discontinuous portions formed by electron beam irradiation on a surface of a steel sheet.